I am curious about the difference between SN754410 and ULN2004 and what are the specific usages of both?
According to data sheets SN754410 is a is a quadruple high-current half-H driver and ULN2004 is a 7 Darlington array. so how should we select these elements in our circuit designs appropriately?

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This question is asking for specific uses of two chips making it too broad to answer and there cannot be a correct answer because it is down to opinion.
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Andy akaMar 24 '14 at 8:21

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Maybe if a context on how to choose between the choices? Such as you need to drive 0.25A Motors or such.
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DeanMar 24 '14 at 9:59

@Andyaka how is asking for a specific use for a type of IC an opinion?
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PasserbyMar 24 '14 at 22:29

2 Answers
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H drivers are usually used for circuits requiring a need to entirely reverse currents to some element. This is usually seen in motor drives. Having an H-drive (or H-bridge) allows one to source current from either side of the motor and sink current from the opposite side. This configuration usually allows one to shut all circuit elements off as well which allows the motor to freewheel so that it's not having to always brake, or accelerate.

The ULN2004 is an array of seven NPN Darlington bipolar transistor pairs, so this is meant to be able to sink current. If it can only sink current, then the other side of circuit element in question (light, motor, or some other high current device) may not be reversed. It can be shut off and turned on only. This is usually not useful in many motor cases where you want reversibility. On the other hand, it's very useful and works fine for devices where polarity doesn't matter: things like LED's which can't be driven backwards anyways or a motor in a device like an electric razor.

The SN754410 is designed to control 4 motors in a single direction, or 2 motors in both directions, with options for enabling or stoping, handling n and p channel transistors/fets, needed resistors, and possibly even flyback/protection diodes (I can't check a datasheet at the moment). It's a complete motor control package.

The ULN2004 is an array of seven NPN Darlington bipolar transistor pairs in a single convenient package to reduce pin count and routing.